1. Field of the Invention
This invention relates to a ceramic-metal composite substrate manufactured by bonding a ceramic base material and a metal material, and on which a semiconductor element, for example, is to be mounted. More particularly, it is concerned with a structure of such substrate capable of preventing the semiconductor element and the ceramic base member from breaking. This invention is also concerned with a method for producing the ceramic-metal composite substrate.
2. Discussion of Background
FIG. 5 of the accompanying drawing is a cross-sectional view illustrating a composite substrate for mounting thereon a semiconductor element, as disclosed in, for example, Japanese Unexamined Patent Publication No. 155580/1985, in which a conventional ceramic base and metal members are directly bonded. In the drawing, a reference numeral 1 dsignates an alumina member as the ceramic base; reference numerals 2A, 2B denote the metal members formed on the ceramic base member, such as, for example, tough pitch electrolytic copper plate to form an electric circuit; and numerals 7A, 7B refer to bonding surfaces, at which the alumina base 1 and the respective copper plates 2A, 2B are directly bonded.
FIG. 6 of the accompanying drawing is a perspective view showing one embodiment of using a conventional substrate for mounting thereon a semiconductor element, the substrate being of a modular structure, on which a semiconductor element is mounted. In the drawing, a reference numeral 4 designates a semiconductor, a numeral 5 refers to solder for mounting the semiconductor element 4 onto a copper plate 2b, and reference numerals 6a, 6b denote bonding wires of, for example, aluminum for driving the semiconductor element 4, the bonding wires being connected to other copper plates 2a, 2b (external connecting electrodes) in a manner to be electrically insulated from the copper plate 2b.
When the modular semiconductor element, particularly a high-power semiconductor element of the above-described construction is actuated, the semiconductor element 4 generates a large amount of heat. As a matter of course, this modulus semiconductor element is used repeatedly, hence the substrate for mounting the semiconductor element is required to have its properties as follows: (i) it can sufficiently dissipate heat to be generated from the semiconductor 4; (ii) it does not give damage to the semiconductor element 4 due to expansion and contraction of the substrate caused by the heat-cycle to be accompanied by actuation and inactuation of the semiconductor element; and (iii) further, the alumina base member itself is not damaged by this heat-cycle.
With the substrate structure as mentioned above, however, the ceramic base 1 generally possesses a low thermal expansion coefficient, which is 7.times.10.sup.-6 for the alumina ceramic in the above-described embodiment. On account of this, when it is brought into direct contact with the copper plates 2A, 2B having the thermal expansion coefficient of 17.times.10.sup.-6, stress occurred in the vicinity of the bonding surfaces 7A, 7B owing to difference in the thermal expansion coefficient between the ceramic base member and the copper plates. When such bonded members are subjected to the heat-cycle, there takes place a point of problem such that a large stress repeatedly occurs in the vicinity of the above-mentioned bonding surfaces 7A, 7B, whereby the hard but brittle alumina base member 1 can no logner withstand such large stress, brings about cracks, and becomes-finally broken.
FIG. 7 of the accompanying drawing is a cross-sectional view showing typical cracks 8A, 8B, 8C and 8D to occur in the alumina base member 1, wherein the cracks start from the corner parts of the ceramic base member 1 and the copper plates 2A, 2B where the stress is concentrated. In addition, since the copper plates 2A, 2B are rigidly bonded to the alumina base member 1, the thermal expansion coefficient thereof is smaller than that of simple copper. Nevertheless, when the silicon semiconductor element 4 having its thermal expansion coefficient of as small as 5.times.10.sup.-6 is mounted on this alumina base member 1 by means of soldering, there still remains a problem such that cracks also occur in the semiconductor element 4 itself. These points of problem emerged conspicuously when the copper plates 2A, 2B were made thicker for increasing the operating current in the semiconductor element 4, and when a semiconductor element having a large area was mounted on the base member.
While it is possible to attain improvement to some extent with regard to the above-mentioned occurrence of the cracks by increasing the thickness of the alumina base member 1, the heat-dissipating property of the semiconductor 4 becomes deteriorated due to high heat-resistance of the alumina base member 1. For example, by increasing the thickness of the alumina base member 1 from its original gauge of 0.4 mm to 0.63 mm, its heat-cycle resistant property in a range of from -40.degree. C. to 150.degree. C. can be increased by about 1.2 times; on the contrary, however, the heat-resistance value of the alumina base member, which hinders heat-dissipation, increases by about 1.6 times. As the consequence, this increase in the thickness of the base plate cannot be so effective a method, when taking the function of the semiconductor element 4, the production cost of the ceramic base member 1, and so forth into consideration.
In order therefore to avoid these problems, various measures need to be taken such that the operating power and the shape of the semiconductor element 4 should be limited, the copper plates 2A, 2B should be made thinner and broader so as to decrease its density for mounting on the base member; and others. However, these measures have stood as great obstacles against increase in the function and the density of the semiconductor module.
Incidentally, as a method for relaxing the internal stress caused by difference in the thermal expansion in a structural material wherein ceramic and steel are bonded together, there has been proposed one, in which a layer of relatively soft metal such as aluminum, copper, etc. or a layer of niobium, or a laminated intermediate layer such as niobium/molybdenum, niobium/tungsten, etc. is provided on the bonding surface of both ceramic and steel to a thickness of a few millimeters (vide: a monthly periodical of a title "Kinzoku (Metal)", May, 1986, pp. 45-50). However, as mentioned in the foregoing, the provision of the intermediate layer having a thickness of a few millimeters on the substrate for mounting the semiconductor element would inevitably deteriorate the heat-dissipating property of the substrate with the consequent problem of inability to increase the capacity and the function of the resulting semiconductor module.